Recently CCDs have been reduced in pixel size as video cameras become smaller and CCDs have more pixels. CCDs smaller in pixel size are reduced in sensitivity and S/N as a natural result. Particularly it becomes difficult to secure picture quality at low illumination. Hence, noise reduction is essential to the latest video cameras.
The configuration of a conventional imaging apparatus will be described below. A structural example of the conventional imaging apparatus is illustrated in FIG. 5. FIG. 5 shows an example of a 3CCD camera which has a CCD 1 composed of three CCDs corresponding to RGB signals, respectively. Three RGB systems are present from the CCD 1 to a γ correcting section 5. To simplify the drawing, thick arrows represent RGB signals and thin arrows represent luminance signals in FIG. 5. The output of the CCD 1 is subjected to proper gain control and proper tone correction according to subject conditions in a gain adjusting section 2 and the γ correcting section 5.
Incidentally, images taken by a video camera decrease in resolution because of high frequency components which are degraded by the characteristic of a lens, the number of pixels constituting the CCD 1, and an aperture ratio of a pixel. Thus, operations of increasing a resolution are performed in a first matrix 7, a second matrix 8, a detail extracting section 9, and an adder 10.
In the first matrix 7, a first luminance signal γ is determined from an RGB signal by the operation of (Formula 1) below.Y=0.30R+0.59G+0.11B  (Formula 1)
In the second matrix 8, a second luminance signal Y′ is determined from an RGB signal by the operation of (Formula 1) or (Formula 2).Y′=0.25R+0.50G+0.25B  (Formula 2)
The reason why Y′ is determined in addition to Y is that in the 3CCD camera when a spatial pixel shift method is used, in which a CCD for G is shifted by a half pixel pitch from CCDs for RB, a loop component for G and a loop component for RB are canceled that are inverted in phase in a frequency region. In this case, the coefficients of RGB in the operation of (Formula 2) are set in such a way that the coefficient of G is 0.5 and the sum of the coefficients of R and B is 0.5.
In the detail extracting section 9, the two-dimensional high frequency component of the second luminance signal is extracted. The second luminance signal serves as the output of the second matrix 8. FIG. 6 shows an example of the internal configuration of the detail extracting section 9. FIG. 7 shows the signal wave form of each part. The waveforms of FIG. 7 indicate vertical and horizontal changes. The following explanation will be made in accordance with FIGS. 6 and 7.
In line memories 11 and 12, an inputted video signal is delayed by 1H (H: horizontal scanning period which is about 63.5 μsec in the NTSC). The output of the line memory 11 is inputted to the line memory 12. As a result, the outputs of the line memories 11 and 12 are equivalent to input signals delayed by 1H and 2H. In a vertical HPF (High Pass Filter) 13, a high frequency component Hv in the vertical direction is determined by the operation of (Formula 3).Hv=(−1+2·H−1−H−2)/4  (Formula 3)
H−1 represents a delay of 1H. Similarly in a horizontal HPF 14, a high frequency component HH in the horizontal direction is determined by the operation of (Formula 4).HH=(−1+2·Z−1−Z−2)/4  (Formula 4)
Z−1 represents a delay of one pixel in the horizontal direction. On the assumption that input to the detail extracting section 9 is a signal indicated in FIG. 7(a) the vertical HPF 13 and the horizontal HPF 14 have outputs where high frequency components are extracted as shown in FIG. 7(b). At this point of time, noise in a signal generally contains a large number of high frequency components and thus noise contained in input to the detail extracting section 9 is also mixed with the outputs of the vertical HPF 13 and the horizontal HPF 14.
In gain adjusting sections 15-1 and 15-2, the outputs of the vertical HPF 13 and the horizontal HPF 14 are each multiplied by a proper gain.
At this point of time, the outputs of the gain adjusting sections 15-1 and 15-2 also act as signals containing much noise as shown in FIG. 7(b). Hence, coring is performed on such a signal by coring sections 16-1 and 16-2, so that noise is removed.
In the coring sections 16-1 and 16-2, as shown in FIG. 8, a part having an input amplitude equal to or smaller than an equivalence th is set at 0 with respect to the outputs of the gain adjusting sections 15-1 and 15-2. The equivalence is subtracted from an input exceeding the equivalence and then output is made. Thus, noise is removed from the output waveforms of the coring sections 16-1 and 16-2 as shown in FIG. 7(c).
In an adding section 17, the output signals of the coring sections 16-1 and 16-2 are added to obtain a two-dimensional high-frequency component.
Referring to FIG. 5 again, in the adding section 10, high frequency components are supplemented by adding the output of the detail extracting section 9 to the output of first matrix means, detail is enhanced as shown in FIG. 7(d), and thus a signal with a high resolution is obtained, though an S/N is somewhat degraded.
Subsequently in a three-dimensional NR (Noise Reduction) 6, field recursive noise reduction is performed on the output of the adding section 10 in the time direction. The field recursive noise reduction will be described below. FIG. 9 shows a structural example of the three-dimensional NR. In a first subtracting section 18, the output of a memory 19 is subtracted from an input signal.
In this configuration, on the assumption that the memory 19 provides a delay of 1V (V: vertical scanning period which is 1/59.94 sec in the NTSC), the output of the first subtracting section 18 has a difference of video signals in two successive fields and thus a change (movement) and noise of a video signal of the 1V period is contained.
In a nonlinear processing section 20, noise is extracted from the output of the first subtracting section 18. Under a general idea of noise being smaller in amplitude than a signal, when the amplitude of an input signal has an absolute value smaller than a threshold value p as shown in FIG. 10, an input is outputted as it is, and an output amplitude is reduced as an absolute value of the amplitude of an input signal becomes larger than the threshold value p. Thus, noise can be extracted from the input signal. Additionally, in the example of FIG. 10, when the amplitude of the input signal has an absolute value equal to or higher than a threshold value q (|q|>|P|), the input is not outputted.
In a second subtracting section 21, the output of the nonlinear processing section 20 is subtracted from the input signal. As a result, the output of the second subtracting section 21 obtains video signal output from which noise is reduced.
The following will describe the relationship between the threshold value p of the nonlinear processing section 20 and noise amplitude. Noise having an amplitude equal to or smaller than the threshold value p passes through the nonlinear processing section 20 and is subtracted from the input signal in the second subtracting section 21, so that noise can be almost completely removed. On the other hand, noise having an amplitude exceeding the threshold value p is attenuated in the nonlinear processing section 20. Thus, even when a subtraction is made from the input signal in the second subtracting section 21, noise cannot be completely removed but partially or totally remains.
The output of the second subtracting section 21 is also inputted simultaneously to a field memory 19 and is used to perform an operation for an input signal after 1V. The output of the three-dimensional NR 6 acts as the output of a camera luminance signal.
The conventional imaging apparatus reduces noise in the above-described manner.
However, the imaging apparatus configured thus has the following problem: when the detail gain of the detail extracting section 9 is increased or the equivalence th of the coring sections 16-1 and 16-2 is reduced in order to increase a resolution, noise amplitude increases in the output of the adding section 10, degrading S/N.
In contrast, when the degradation of S/N is corrected by the three-dimensional NR 6, it is necessary to increase the equivalence p of the nonlinear processing section 20. When the equivalence p is increased, a change of a signal is also contained in the output of the nonlinear processing section 20. Thus, the output of the second subtracting section 21 causes a serious degradation of an afterimage. Moreover, when the equivalence th of the coring sections 16-1 and 16-2 is increased, the detail with a small amplitude disappears, resulting in an entirely blurred image, though S/N is improved.